1. Field of the Invention
The present invention relates to a method of growing a single crystal in low-temperature regions in a semiconductor film that is deposited on a substrate, and a semiconductor device fabricated using such a semiconductor film with a grown single crystal.
2. Description of the Related Art:
Static random-access memories (SRAMs) of the high-resistance-load type fabricated using a semiconductor film deposited on a substrate comprise load-type memory cells that are fabricated of a polycrystalline semiconductor, i.e., polycrystalline silicon. However, it is difficult for the SRAMs of the high-resistance-load type to maintain sufficient levels of operating margin, reliability, standby current.
To solve the above problem, there has been proposed a laminated SRAM using, as load elements, thin-film transistors formed in a polycrystalline semiconductor of highly uniform film qualities, e.g., polycrystalline silicon.
Various processes have heretofore been proposed for fabricating a polycrystalline semiconductor to manufacture such a thin-film semiconductor device.
The proposed processes include a chemical vapor deposition process, a random solid-phase growing process, and a process of selectively forming a single-crystal region.
One conventional example of the process of selectively forming a single-crystal region will be described below with reference to FIGS. 1A through 1C of the accompanying drawings.
As shown in FIG. 1A, a low dose (for example, 1.times.10.sup.14 cm.sup.-2 at 40 KeV) of silicon ions (Si.sup.+) is introduced into a polycrystalline silicon layer 3 on a silicon oxide layer 2 by ion implantation. Then, as shown in FIG. 1B, a resist mask 20 is deposited on the polycrystalline silicon layer 3, and a high dose (for example, 2.times.10 .sup.15 cm.sup.-2 at 40 KeV) of silicon ions (Si.sup.+) is introduced selectively into the polycrystalline silicon layer 3 selectively in those regions which are not covered with the resist mask 20. Thereafter, the resist mask 20 is removed, and, as shown in FIG. 1C, crystals are grown in the ion-implanted regions by a low-temperature solid-phase growing process at a temperature of 600.degree. C. for 20 hours, thus producing single-crystal silicon regions 6.
However, fabrication of the above thin-film semiconductor device poses various problems as described below:
(1) If a polysilicon film is formed of large crystal grain according to the normal chemical vapor deposition process, then the film qualities suffer a lack of uniformity, making it difficult to fabricate a polycrystalline semiconductor film with high electron mobility at low leakage. PA1 (2) The random solid-phase growing process allows fabrication of a polycrystalline semiconductor film of a large grain size of 1 .mu.m or greater. Since, however, it is difficult to grow the single crystal grain selectively in a desired position, a desired transistor cannot easily be fabricated in the desired position regardless of the large grain size available. The solid-phase growing process makes it difficult to form a single-crystal layer in a wide area because of the presence of minute defects. If a transistor were formed in such a single-crystal layer, then its channel would be located in the grain boundary or crystal defects would be present in the channel, resulting in low reliability due to an increased leak current or large variations of a threshold voltage Vth. PA1 (3) To produce SOI (Silicon On Insulator) or SOS (Silicon On Sapphire) arrangements, there have been proposed an argon-laser application process, a zone-melt process, and a process of bonding a silicon-crystal semiconductor substrate to an insulative substrate and thereafter grinding the silicon-crystal semiconductor substrate to a desired thickness, thereby producing a thin silicon-crystal semiconductor film. However, these processes under poor reproducibility and low throughput. PA1 (4) Many research efforts have been directed in recent years to the fabrication of a thin silicon-crystal semiconductor film using a laser for applying a pulsed UV (ultraviolet) radiation in a plane, i.e., an excimer laser. Since the ultraviolet radiation is absorbed by silicon, such a process is considered effective in forming a semiconductor film on a glass substrate, for example. Actually, however, only a thin polycrystalline film is formed in many occasions.